This application is a divisional of U.S. patent application Ser. No. 07/502,793 filed Apr. 2, 1990, now abandoned, which is a continuation-in-part of U.S. Pat. application Ser. No. 07/357,794, filed May 30, 1989 now U.S. Pat. No. 4,938,285, which is a continuation-in-part of U.S. patent application Ser. No. 211,567, filed Jun. 27, 1988, now U.S. Pat. No. 4,858,688.